This invention relates to an optical disk capable of reproducing and/or recording information.
Optical disks have been developed which may be applied to various fields as information carriers, from which information may be read out or reproduced and onto which information may be recorded. Such optical disks may include, for example, reproduction-type disks, such as the so-called compact disks for digital audio disk systems; video disks for optical video disk systems; and recording/reproduction-type disks for video file systems or computer output memory systems.
In such optical disks, a tracking guide is preformed before recording information onto the optical disk for high-density recording, and pre-pits or preformation pits, such as codes representing the track number and sector number, are formed to specify the recording or readout region. In a conventional information recording/reproducing system, the track number and sector number are read out from the pre-pits while tracing the tracking guide with a laser beam. Subsequently, information pits are formed in the tracking guide and information is read out therefrom.
In the conventional optical disk, tracking guides are discontinuously or intermittently formed along its circumference. Flat regions between the circumferential tracking guides are defined as regions in which the preformation pits are to be formed. The preformation pits are formed to have a depth of substantially .lambda./4 to obtain a highest detection sensitivity, where .lambda. is the wavelength of the laser beam. In the conventional optical disk of the type described above, an area having the tracking guide can be properly traced with the laser beam. However, a preformation pit area between adjacent tracking guides is traced using the preformation pit array as the tracking guide. However, the laser beam spot tends to deviate from the preformation pit array. This is because a symmetrical pattern is formed on a detector when the edges of preformation pit array or the center thereof is traced with the laser beam spot and the detector may erroneously detect the edges of the pit array as the center thereof. It is known that preformation pits having a depth of .lambda./4 are suitable for properly reading out information from the optical disk, and that the pit array cannot be properly used as the tracking guide.